Abstract

Artificial percepts (phosphenes) can be induced by applying transcranial magnetic stimulation (TMS) over human visual cortex. Although phosphenes have been used to study visual awareness, the neural mechanisms generating them have not yet been delineated. We directly tested the two leading hypotheses of how phosphenes arise. These hypotheses correspond to the two competing views of the neural genesis of awareness: the early, feedforward view and the late, recurrent feedback model. We combined online TMS and EEG recordings to investigate whether the electrophysiological correlates of conscious phosphene perception are detectable early after TMS onset as an immediate local effect of TMS, or only at longer latencies, after interactions of TMS-induced activity with other visual areas. Stimulation was applied at the intensity threshold at which participants saw a phosphene on half of the trials, and brain activity was recorded simultaneously with electroencephalography. Phosphene perception was associated with a differential pattern of TMS-evoked brain potentials that started 160-200 ms after stimulation and encompassed a wide array of posterior areas. This pattern was differentiated from the TMS-evoked potential after stimulation of a control site. These findings suggest that conscious phosphene perception is not a local phenomenon, but arises only after extensive recurrent processing.